1. Field of the Invention
The present invention generally relates to a network monitoring technique and, more particularly, to a network monitoring method for monitoring and controlling network constituent elements, such as a transmission apparatus and a router, using an operation system, and a network monitoring apparatus performing such a network monitoring method.
2. Description of the Related Art
Conventionally, network structural elements such as a transmission apparatus and a router that constitute a communication network are monitored and controlled by an operation system.
FIG. 1 is an illustration for explaining an example of a conventional network monitoring method. In FIG. 1, transmission apparatuses 10a1, 10a2 and 10a3 and routers 12a, 12b and 12c, each of which is a network constituent element, constitute a communication network. The routers 12a, 12b and 12c are connected to a monitoring network 14 such as a WAN or LAN. An operation system 16 is connected to the monitoring network 14 through a router 12d. 
The operation system 16 comprises a client system, which receives requests of services such as monitoring and controlling from an operator, and a server system, which executes service requests received by the client system. The operation system 16 monitors and controls the transmission apparatuses 10a1, 10a2 and 10a3 and the routers 12a, 12b and 12c. 
The operation system 16 performs a health check or polling using the ping periodically (for example, every 60 seconds) on each of the transmission apparatuses 10a1, 10a2 and 10a3 and the routers 12a, 12b and 12c that are under management of the operation system 16. Each of the transmission apparatuses 10a1, 10a2 and 10a3 sends a trap to the operation system 16 to notify occurrence of an abnormality when detecting the abnormality of its own. However, such a notification is not made in cases of a communication disconnection of the monitoring network 14, a power supply disconnection of a transmission apparatus, a malfunction of a trap sending part, etc.
It should be noted that Japanese Laid-Open Patent Application No. 2001-28586 discloses a means for constituting one or a plurality of virtual terminals each of which has a physical address and a communication address, and a means for testing one or a plurality of network apparatuses to be tested by simulating an FTP file transfer of TCP/IF and an ICMP-ECHO communication by a virtual terminal.
With the recent improvement in performance of CPUs, a number of transmission apparatuses to be monitored and controlled by one operation system tends to increase. On the other hand, there are many cases in which a monitoring network is not updated even if a number of monitoring and transmission apparatuses is increased since it takes an extremely large cost to update the monitoring network as an infrastructure.
Moreover, as for a method of monitoring from an operation system, detection of malfunction information of a transmission apparatus is performed by a trap from the transmission apparatus and a health check and polling from the operation system. However, since the trap from a transmission apparatus does not reach the operation system when communication through the monitoring network is disconnected, the health check and polling from the operation system are indispensable for monitoring the transmission apparatus.
In such a case, in order to detect immediately an occurrence of a malfunction and recovery thereof, it is necessary to perform a health check or polling at a short period, which increases a load to a monitoring network with an increase in a number of transmission apparatuses.